Implement conversions from `std::fs::Metadata*` types

[multimeric]

For example: impl<T> From<T> for FileAttr where T: std::fs::MetadataExt.

Currently, this is a pretty trivial mapping:

fn metadata_convert(m: Metadata) -> FileAttr {
    FileAttr {
        ino: m.ino(),
        size: m.size(),
        blocks: m.blocks(),
        atime: from_unix_secs_signed(m.atime()),
        mtime: from_unix_secs_signed(m.mtime()),
        ctime: from_unix_secs_signed(m.ctime()),
        kind: match m.file_type() {
            ft if ft.is_file() => FileType::RegularFile,
            ft if ft.is_dir() => FileType::Directory,
            ft if ft.is_symlink() => FileType::Symlink,
            ft if ft.is_block_device() => FileType::BlockDevice,
            ft if ft.is_char_device() => FileType::CharDevice,
            ft if ft.is_fifo() => FileType::NamedPipe,
            ft if ft.is_socket() => FileType::Socket,
            _ => return Err("Unknown file type".into()),
        },
        perm: (m.mode() & 0o7777) as u16,
        nlink: m.nlink().try_into()?,
        uid: m.uid(),
        gid: m.gid(),
        rdev: m.rdev().try_into()?,
        blksize: m.blksize().try_into()?,
        // MacOS only, which we don't yet support
        crtime: UNIX_EPOCH,
        flags: 0,
    }
}

However, we can't implement is using From/Into since we don't own the trait. Moving it upstream would be ideal. I'm happy to provide my implementation.